China published coordinates for the zone on the weekend. The area, about two-thirds the size of the United Kingdom, covers most of the East China Sea and the skies over a group of uninhabited islands at the centre of a bitter row between Beijing and Tokyo.

Japan and its close ally, the United States, sharply criticized the move, which experts said was aimed at chipping away at Tokyo's claim to administrative control over the area, including the tiny uninhabited islands known as the Senkaku in Japan and the Diaoyu in China.

While China said the new rules would not affect "normal operations" for international flights, it added that it would take "defensive emergency measures" against aircraft that failed to identify themselves properly.

China's latest move could help spread the view that Japan was losing administrative control of the area, said Hiroko Maeda, research fellow at Japanese think-tank the PHP Institute.

"China has already been sending its ships (there). It is clear China is trying to undermine Japan's administrative control. Now they are stepping up their effort in the sky as well," Maeda said.

Civil aviation officials from Hong Kong and Taiwan said their carriers entering the zone must send flight plans to Chinese aviation authorities. A transport ministry official in Seoul said South Korean planes would do the same.

An official at the Japan Civil Aviation Bureau said Japanese airlines flying through the region to non-mainland Chinese destinations would likely need to inform China of their plans. "Airlines have been advised to take greater care in the area," said another bureau official.

Singapore Airlines and Qantas Airways Ltd said they would keep Chinese authorities informed of their flights through the area.

Korean Air said its flight plans would be delivered to Chinese authorities but the routes its pilots took would not be affected. Japan Airlines and ANA Holdings also said the zone had not affected their flights.

War of words

Japan protested the weekend move, warning of an escalation into the "unexpected" if Beijing enforced the rules. U.S. Defense Secretary Chuck Hagel called the move a "destabilizing attempt to alter the status quo in the region".

While Washington does not take a position on the sovereignty of the islands, it recognizes that Japan has administrative control over them and is therefore bound by treaty to defend Japan in the event of an armed conflict.

Tensions flared last year between Beijing and Tokyo when the Japanese government bought three of the islands from a private landowner to fend off a potentially more inflammatory purchase by the Tokyo metropolitan government, at the time headed by nationalist governor Shintaro Ishihara.

Asian and Western diplomats said the zone was a problem for Japan, the United States and other countries that may be wary of any acknowledgement of China's claims over the area.

"No one wants to be in a position where by following Chinese instructions you are giving tacit acknowledgement of their sovereignty over a disputed area," one Asian diplomat said. "And there is a fear that is precisely the game that is being played - it seems no accident that the disputed Senkaku islands are now in the heart of overlapping zones."

Japan has its own Air Defence Identification Zone but officials said Tokyo only required aircraft seen to be approaching Japanese territorial airspace to identify themselves.

In its announcement on Saturday, China's Defence Ministry said it would set up other such zones when preparations were finalized. It gave no further details and the ministry's news department declined to elaborate when contacted by Reuters.

China also claims the South China Sea, which is believed to be rich in oil and gas. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also claim parts of the South China Sea, making it one of the region's biggest flashpoints.

China patrols under way

China's official Xinhua news agency said the rules for the East China Sea came into effect on Saturday and that the Chinese air force conducted its first patrol over the zone. The patrol included early warning aircraft and fighters, it said.

Japanese Prime Minister Shinzo Abe said China was forcing other countries to conform to its rules.

Abe, who came to office last December promising to beef up the nation's military, has said Tokyo's door is open to dialogue to improve Sino-Japanese ties but has declined to acknowledge the existence of a formal territorial dispute over the islands, a step upon which Beijing insists.

"It's a unilateral step, changing the status quo in the East China Sea," Abe said in parliament on Monday.

China's Defence Ministry said it was within the country's right to set up the zone.

"We reiterate that the purpose of China's approach is to defend national sovereignty and territorial airspace security, maintain the order of airspace flight, and is an effective exercise of our right of self-defence," spokesman Yang Yujun said in a statement.

Yang said China's move complied with international law.

"The United States, on the issue of the Diaoyu islands, must earnestly not take sides, not make inappropriate remarks and not give the wrong signal to Japan and encourage (its) risky behavior," Yang said.

Japan's own Air Defense Identification Zone extends around the Japanese archipelago and overlaps with China's new zone in part of the East China Sea.

"We might have more risk of encounters with Chinese aircraft in the area," said one former Japanese air force official. "We need to establish a system to avoid unnecessary incidents."

The topic was hot on China's Twitter-like Sina Weibo, with some users calling for war with Japan. "There can be no discussion on territorial issues, only war," wrote one user.